Data transmission is the conveyance of any kind of information from one space to another. Historically this could be done by courier, a chain of bonfires or semaphores, and later by Morse code over copper wires.
In recent computer terms, it means sending a stream of bits or bytes from one location to another using any number of technologies, such as copper wire, optical fibre, laser, radio, infra-red light or even a so called Bluetooth. Practical examples include moving data from one storage device to another and accessing a website, which involves data transfer from web servers to a user's browser.
A related concept to data transmission is the data transmission protocol used to make the data transfer legible. Current protocols favour packet based communication.
Types of data transmission
Serial Transmission bits are sent over a single wire individually. Whilst only one bit is sent at a time high transfer are possible. This can be used over longer distances as a check digit or Parity bit can be sent along it easily.
Parallel Transmission Multiple wires are used and transmit bits simultaneously and is much faster than Serial transmission as one byte can be sent rather than one bit. This method is used internally within the computer, for example the internal buses, and sometimes externally for such things as printers, however this method of transmission is only available over short distances as the signal will degrade and become unreadable, as there is more interferance between many wires than between one, so it degrades.
Speed of transmission
The speed of data transmission is governed by the Baud rate and bit rate. In a summary the bit rate is the rate of data transmission serially, measured in bits per second. The baud and bit rates are usually the same at lower speeds, however the baud rate can be lower but still giving a high bit rate. The baud rate is the number of bits that can be encoded into a single signal per unit of time, e.g.: the volts on a line, to increase bit rate you can encode 0,1,2 or 3 volts rather than 0 and 1 volts.
0 = 00
1 = 01
2 = 10
3 = 11
So the baud rate could be 28kbps but the bit rate would be double at 56kbps.
Asynchronous and synchronous data transmission
Asynchronous transmission uses start and stop bits to signify the beginning and end of a transmission. This means that an 8 bit ASCII character would actually be transmitted using 10 bits e.g.: A would become 1 0100 0001 0. The extra one (or zero depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data is sent intermittently as opposed to in a solid stream. In the following example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognise when the second packet of information is being sent.
Synchronous transmission uses no start and stop bits but instead synchronises transmission speeds at both the receiving and sending end of the transmission using clock signals built into each component. A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in protocol (computing) for sending/receiving data, so some bytes could become corrupted (by losing bits). Ways to get around this problem include re-synchronisation of the clocks and use of check digits to ensure the byte is correctly interpreted and received.